In recent years, mechanical tree harvesters have virtually obsoleted manual harvesting of trees. Many tree harvesting machines are now available from various manufacturers. Usually a tree harvester consists of a harvesting head that is adapted to be mounted on a vehicle and the harvesting head has gripping means for gripping a growing tree and holding the tree while it is severed. Usually these harvesting heads are attached to some type of boom and can be manipulated so that the harvested tree can be placed on the ground in a predetermined oriented position, usually in a stack with other trees.
The rising cost of mechanical harvesters and the decreasing size of trees being harvested has made it desirable to be able to cut several smaller trees and accumulate them on the harvesting head so that a plurality of trees can be harvested before they are stacked since the stacking operation requires substantial manipulation which decreases the productivity of the machine.
One type of mechanical harvester for trees is disclosed in Coffey U.S. Pat. No. 3,664,391. This patent discloses a harvester head which is capable of being universally moved about the end of a boom and has a plurality of grapple assemblies and an accumulator assembly supported thereon. A similar type of tree harvesting apparatus is disclosed in Smith U.S. Pat. No. 3,805,860.
While these types of units have been successful for cutting and accumulating trees when the size of the trees is significantly smaller than the maximum size tree which the felling head is capable of cutting, these units are not capable of cutting more than one tree and accumulating trees at or near the maximum size tree for the felling head. Thus, when trees approaching the maximum size for the felling head is being harvested, each individual tree must be removed from the feller head before a subsequent tree can be harvested.
Other attempts have been made to design feller-buncher heads which are capable of severing and accumulating larger trees, approaching the maximum size for the feller-buncher head. Examples of such apparatus are disclosed in Larson et al. U.S. Pat. No. 3,468,352 and Hamilton U.S. Pat. No. 3,749,143. While these units are designed to be capable of cutting and accumulating several trees on the accumulator head, the designs have an inherent shortcoming which has limited the acceptability of such units. The devices disclosed in the two patents have the inherent disadvantage of not having a positive gripping action on the tree while it is being severed. In both types of units disclosed in these patents, the gripping force on the growing tree relies upon a resilient member which defines part of the gripping force thereby minimizing the control which the operator has over the gripping force on the tree as it is being severed.